This invention relates to an idling rpm feedback control method for internal combustion engines, and more particularly to a method of this kind which is intended to prevent engine stall when the engine is operated in a low atmospheric pressure condition, such as at a high altitude.
In an internal combustion engine, the engine can easily stall due to a drop in the engine speed when the engine is operated in an idling condition at a low temperature of the engine cooling water or when the engine is heavily loaded with electrical loads by head lamps, an electric fan, etc. in a vehicle equipped with the engine. To eliminate such disadvantage, an idling rpm feedback control method has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 55-98628, which comprises setting desired idling rpm in dependence upon load on the engine, detecting the difference between the actual engine rpm and the desired idling rpm, and supplying supplementary air to the engine in a quantity corresponding to the detected difference so as to minimize the same difference, to thereby control the engine rpm to the desired idling rpm.
According to this proposed method, however, if the clutch or the transmission gear of the engine is disengaged while the engine is decelerating with the throttle valve fully closed, the engine speed can suddenly drop, depending upon the magnitude of load applied on the engine. Even if the above idling rpm feedback control is initiated immediately after such a sudden drop in the engine speed, it cannot promptly increase the supplementary air quantity at a rate sufficient to prevent a further drop in the engine speed, often causing engine stall. Even in the event that the engine is decelerated into the feedback-mode controlling region without the clutch or the transmission gear being disengaged during the deceleration, there can occur a delay in the supply of a quantity of supplementary air required for maintaining the engine speed at the desired idling rpm, also resulting in a drop in the engine speed to even cause engine stall depending upon the magnitude of the engine load, since only a quantity of supplementary air is supplied to the engine at the start of the feedback control, which just corresponds to the difference between the actual engine rpm and the desired idling rpm. In order to avoid this disadvantage, a method has been proposed by Japanese Provisional Patent Publication No. 55-1455, which comprises supplying a predetermined quantity of supplementary air to the engine in advance of initiation of the feedback control when the engine speed decreases below a predetermined value at deceleration of the engine, and a method has also been proposed by Japanese Provisional Patent Publication (Kokai) No. 55-98629, which comprises previously supplying the engine with supplementary air in such a manner that the supplementary air quantity is gradually increased as the engine speed decreases until it reaches a predetermined amount, for a period of time starting from the time the engine speed has decreased below a predetermined value at deceleration of the engine and until the feedback control is initiated.
However, when the engine is operated in a place where the ambient atmospheric pressure is low, such as at a high altitude, the mass flow of intake air supplied to the engine per one suction stroke of same is smaller than that when the engine is operated under standard atmosphere. As a consequence, if the engine is supplied with supplementary air at a volumetric flow rate set to a value appropriate to a standard atmospheric pressure condition, during deceleration under such a low atmospheric pressure, a shortage of the intake air will take place to cause a drop in the engine speed, and even engine stall depending upon the magnitude of the engine load. To avoid this disadvantage, the supplementary air should be supplied to the engine at an increased volumetric flow rate so as to make the mass flow of intake air supplied to the engine per one suction stroke under such a low atmospheric pressure equal to that under a standard atmospheric pressure.